meter. It was 19,2357: we re-melted it, and its specific gravity became 19,2240. After having polished it, its specific gravity was 19,2390: we then struck it, and its specifie gravity became 19,2487.

Piece of silver

Piece of re-melted silver

Piece of silver struck..

Piece of copper

Piece of copper, struck..

Piece of copper, struck a second time

If we compare the foregoing experiments, we see that, independently of the difference of the specific gravities of the metals, gold undergoes in the compression caused by the shock a less condensation than silver, and the latter in its turn less than copper; and that the heat extricated is in proportion to the change of dimension. But we must compare the piece of copper with that of silver and of gold, regarding all three in the state in which they are when they are reduced into plates, i. e. when they have undergone the pressure of a flatting-mill, because copper cannot acquire its greatest dilatation by re-melting, which alters its surface. The re-melting, by producing a greater state of dilatation, diminishes the tenacity, which was owing to the compression of the metal, and it increases at the same time the proportion of caloric, which varies with the

dimensions.

We struck into moulds some pieces of similar dimension to the above; but the heat extricated was less considerable, and the specific gravity received less increase than when the pieces struck were at liberty. This was certainly owing to the particles in the latter case coming more closely together by sliding over one another.

At the commencement of our experiments we made use of copper, and had, as we supposed, clearly ascertained that heat was extricated by compression; but, when all circumstances were alike, instead of heat, we had a production of cold. We verified the temperature of all the objects which could have any influence, and we found, by employing a very accurate thermometer of M. Pictet's, which we had used throughout the experiments, that the stamper of the flypress was nearly half a degree below the temperature of the copper: we directed one experiment towards this object, and we ascertained that the communication of temperature takes place in a much more rapid manner by means of the blow and of compression, than when the bodies are

simply in contact. Since then, we took every necessary precaution in order that the fly-press and the metallic pieces should be at the same temperature before making the flypress act.

It results from what precedes, that the heat which is produced by compression in bodies which do not undergo any chemical change, is merely owing to the changes of dimension which these bodies undergo; and when the dimensions can no longer be diminished, the shock, however violent does not cause any heat: solids then become similar to liquids, which may undergo violent and repeated shocks without any change in temperature: for it appears to me to be natural to attribute the small extrication of heat which we have been able to observe in the pieces which had undergone three operations, either to a small condensation which might still be produced in them, or to the effects of the elastic particles of the fly-press, which had been able to re-adjust themselves after the shock. 2dly, That the communication of the heat takes place much more rapidly by a strong compression than by simple contact: from which it follows, that in our experiments we have been able to obtain but a small part of the effects of the extrication of heat produced by compression; but this part ought to be in relation with the total effect.

X. On the Parts of Trees primarily impaired by Age. In a Letter from T. A. KNIGHT, Esq. F.R.S. to the Rt. Hon. Sir JOSEPH BANKS, Bart. K.B. P.R.S.*

MY DEAR SIR, IN the first communication I had the honour to address to you, (it was in the year 1795,) I stated the result of many experiments on grafted trees, from which I inferred that each variety can be propagated with success, during a limited period only; and that the graft, or other detached part of an old tree, or old variety, can never form that which can with propriety be called a young tree.

I have subsequently endeavoured to ascertain which, amongst the various organs that compose a tree, first fails to execute its office, and thus tends to bring on the incurable debility of old age; and the result of the experiments appears sufficiently interesting to induce me to communicate an account of them to you.

Whatever difference exists between the functions of ani

* From the Philosophical Transactions for 1810, Part II.

mal

curacy.

mal and vegetable life, there is a very obvious analogy be tween some of the organs of plants, and those of animals; and it does not appear very improbable, that the correspondent in each, may organ, first fail to execute its office; and satisfactory evidence of the imperfect action of any particular organ can much more easily be obtained in the vegetable than in the animal world. For a tree may be composed, by the art of the grafter, of the detached parts of many others; and the defective, or efficient, operation, of each organ, may thus be observed with the greatest acBut such observations cannot be inade upon animals; because the operations necessary cannot be performed; and therefore, though there would be much danger of error in incautiously transferring the phænomena of one class of organized beings to another, I conceive that experiments on plants may be, in some cases, useful to the investigator of the animal economy. They may direct him in his pursuits, and possibly facilitate his inquiries into the immediate causes of the decay of animal strength and life; and on a subject of so much importance to mankind, no source of information should remain unexplored, and no lights, however feeble, be disregarded.

Naturalists, both of ancient and modern times, have considered the structure of plants, as an inversion of that of animals, and have compared the roots to the intestines, and the leaves to the lungs, of animals; and the analogy between the vegetable sap, and animal blood, is very close and obvious. The experiments also, of which I have at different periods communicated accounts to you, supported by the facts previously ascertained by other naturalists, scarcely leave any reasonable grounds of doubt, that the sap of trees circulates, as far as is apparently necessary to, or consistent with, their state of existence and growth.

The roots of trees, particularly those in coppices, which are felled at stated periods, continue so long to produce, and feed, a succession of branches, that no experiments were wanted to satisfy me, that it is not any defective action of the root which occasions the debility and diseases of old varieties of the apple- and pear-tree; and indeed experience every where shows, that a young seedling stock does not give the character of youth to the inserted bud or graft. I however procured plants from cuttings of some very old varieties of the apple, which readily emit roots; and these plants at the end of two years were grafted, about two inches above the ground, with a new and very luxuriant variety of the same species. These grafts grew very freely, and the

roots

roots themselves, at the end of four or five years, probably contained at least ten times as much alburnum, as they would have contained had the trees remained ungrafted. The roots were also free from every appearance of disease or defect.

Some crab-stocks were at the same time grafted with the golden pippin, in a soil where the wood of that variety rarely lived more than two years; and I again grafted the annual shoots of the golden pippin with cuttings of a young and healthy crab-tree; so as to include a portion of the wood of the golden pippin between the roots and branches of the native uncultivated species, or crab-tree; and in this situation it grew just as well as the wood of the stock and branches. Some branches also of the golden pippin trees, which I mentioned in my former,communication of 1795, being much cankered, were cut off about a foot above the junction of the grafts to the stocks, and were regrafted with a new and healthy variety. Parts of the wood of the golden pippin, in which were many cankered spots, were thus placed between the newly inserted grafts, and the stocks; and these parts have subsequently become perfectly free from disease, and the wounds, previously made by canker, have been wholly covered with new and healthy bark. These facts, therefore, satisfied me, that the debility and diseases of old varieties of fruit of this species, did not originate in any defective action of the bark or alburnum, either of the root, or of the stem and branches; and my attention was consequently directed to the leaf and succulent animal shoot.

A few crab stocks were grafted with cuttings of the golden pippin, in a situation and soil where I had previously ascertained that the wood of the golden pippin rarely remained in health at the end of a second year; and, as soon as the annual shoots had acquired sufficient growth and firmness, numerous buds of a new and luxuriant variety of apple, which had recently sprung from seed, were inserted in them. During the succeeding winter, the natural buds of the golden pippin branches were destroyed, and those inserted suffered alone to remain; and as soon as the leaves of these had unfolded, and entered on their office, every symptom of debility and disease disappeared in the bark and wood of the golden pippin; and each continued to perform its office, just as well as the wood and bark of the young seedling stocks could have done under similar circumstances. I made nearly the same experiments on the pear-tree, and with the same result.

I have endeavoured, in several former communications,' to prove that the sap of plants circulates through their leaves, as the blood of animals circulates through their lungs; and I have not subsequently found any facts, in the writings of other naturalists, or in my own experiments, which militate against this conclusion. I have also observed, that grafted trees, of old and debilitated varieties of fruit, became most diseased in rich soils, and when grafted on stocks of the most vigorous growth; which has induced me to suspect, that in such cases more food is collected, and carried up into the plant, than its leaves can prepare and assimilate, and that the matter thus collected, which would have promoted the health and growth in a vigorous variety, accumulates, and generates disease in the extremities of the branches and annual shoots, whilst the lower part of the trunk and roots remain, generally, free from any apparent disease. I am, therefore, much disposed to attribute the diseases and debility of old age in trees, to an inability to produce leaves, which can efficiently execute their natural office; and to some consequent imperfection in the circulating fluid. It is true that the leaves are annually reproduced, and therefore annually new but there is, I conceive, a very essential difference between the new leaves of an old, and of a young variety: and in support of this opinion, I shall observe, that the external character of the leaf of the same variety at two, and at twenty years old, is very dissimilar; and it therefore appears not improbable, that further changes will have taken place at the end of two centuries *.

If these opinions be well founded, and the leaves of trees be analogous to the lungs of animals, is it very improbable that the natural debility of old age of trees and of animals, may originate from a similar source?-This is a question, upon which I am not by any means prepared to give an opinion but I believe it will very generally be admitted, that the human subject is best formed for long life, when the chest is best formed to permit the lungs to move with most freedom. I have also long and attentively observed

The leaf of a seedling apple or pear-tree, when the plant is very young, is generally almost wholly free from the pubescence or down, which subsequently appears on its under surface; and which Bonnet and M. Mirbel have supposed to increase its surface and powers. But I feel little disposed to adopt this hypothesis, having observed that the leaves of some new varieties of the apple, which have sprung from seeds of the Siberian crab, have both surfaces nearly equally smooth; and that these varieties grow faster, and bear heavier crops of very rich fruit, than any others, without being exhausted or injured.

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